This invention relates to power transmission devices and is directed in particular to such devices in which there is a continuously variable input-output speed ratio capability. Furthermore, this invention is directed to the above type of device in which the power transmitting characteristics are particularly favorable in connection with automotive use, although it is to be understood that the invention is not limited to such use.
It has long been recognized that vehicle transmissions employing planetary gear sets offer many advantages over other types. The multiple tooth contact and symmetrical distribution of loading provide efficient use of material, compactness and ruggedness of planetary gear sets. They also lend themselves to constructions in which the gear ratio changes may be effected by hydraulic actuators and when coupled to the input shaft through hydraulic torque converters offer a degree of smoothness and operation which has led to their virtual universal use as a standard power transmission for automotive and similar vehicles.
The inherent advantages of this already developed transmission technology has been integrated with an infinitely variable hydraulic drive to provide an improved continuously variable transmission. The infinitely variable drive may include any type of positive controllable drive such as a hydrostatic pump-motor circuit. A typical approach utilizes a dual mode operation employing a single input drive to the planetary for one mode or range of operation and a dual input drive to the planetary for another mode or range of operation and having synchronous shifting capability between the two modes. Examples of this operation are presented in my prior U.S. Pat. No. 3,306,129, Feb. 28, 1967, and its Reissue U.S. Pat. No. Re 27,307, Mar. 14, 1972, and Ross U.S. Pat. No. 3,396,607, Aug. 13, 1968.
In my prior patent and its reissue U.S. Pat. No. Re 27,307, there is shown in FIG. 8 thereof a transmission which operates, in one range, as a split power path transmission having one power path directed through an infinitely variable hydrostatic drive to one element of a high speed planetary gear set and a second power path of the mechanically coupled type directed to another element of the high speed planetary gear set. When operated thusly, this transmission is in its high speed mode of operation and when in this mode its range is extended by virtue of functioning the hydrostatic drive to impart input rotation first in one direction of rotation and later in the opposite direction of rotation. For reverse and low speed operation, the ring gear of a low speed planetary gear set is braked and single drive input through the hydrostatic drive unit is applied to the sun gear of the low speed planetary set. In this way, the torque multiplying characteristic available from the low speed planetary gear set is utilized in the low speed mode.
In the Ross U.S. Pat. No. 3,396,607, another approach is used. In this case, a planetary gear set is used to receive dual input drive thereto in high speed mode, one input being through a hydrostatic unit to the ring gear and the other input is a mechanical drive to the planet carrier. In low range, this same planetary gear set is locked out (the ring gear is coupled directly to the planet carrier) so that it functions as a direct coupling and not as a planetary system whereby the inherent torque multiplication of the planetary gear set is not realized.
In designing any planetary gear system for a transmission, it is well understood that there are practical limits on the relative sizes of the sun and planet gears. For example, the maximum size ratio between the sun gear and planet gears is limited to about 2.5/1 otherwise the relative planet gear diameter becomes so small that there is no practical way to support the planet gears on individual shafts and bearings. At the other end of the scale, if the sun gear is of small size with relation to the planet gears difficulties are also encountered. For example, if the size ratio between the sun gear and planet gears is 1/2.5, the sun gear becomes so small that its load carrying capacity and support become completely impractical. The choice of which of these gears is to be larger of course depends upon the selection of which element of the planetary is to receive the drive input, which is to deliver the drive output and which is to be braked. Accordingly the gear sizes, the basic planetary kinematics, practical limits of the number of gear teeth and power path through the planetary impose stringent design limits on the system.
If, now, one imposes upon this design the further requirement that it shall be capable of dual mode operation, provision must be made for the fact that in one mode there will be a single input whereas in the other mode there will be dual input to the planetary system. Within this further constraint, the practical difficulties are multiplied if one is to achieve the requisite torque multiplication in low and reverse speed mode and also the requisite extended range in high speed mode.
Still further complications arise if the dual mode transmission is also desired to allow for synchronous shifting between modes.
Synchronous mode or range operation imposes additional design limitations on the planetary gear system. In addition to providing the operating drive ratios the planetary must further be constructed to provide that when operating in one mode that the connecting drive for the other mode of operation be rotating in the same direction and at the same speed as the member to which it is to be connected.
To achieve the desired dual mode, extended high speed range and synchronous shifting functions in my prior U.S. Pat. No. 3,306,129 and its Reissue U.S. Pat. No. Re 27,307, as shown in FIG. 8, the single input mode is through the main shaft to the sun gears and the dual input mode employs a second shaft which is in spaced, parallel relation to the main shaft and connects to the forward carrier member.
In the Ross U.S. Pat. No. 3,396,607 the dual drive connection is also through parallel shafts. However, since this planetary is nonfunctional in the single input or low range mode (i.e. it is locked and no torque multiplication is effected) the necessary torque multiplication must and is provided by gear reduction from the power planet to the single input drive to the locked planetary.
These and similar prior art transmissions have a common construction where the single and dual drive connections are necessarily spaced on parallel drive shafts in order to effect the necessary driving speeds, direction of rotation and connection points on the planetary gear system. From a practical point of view, such arrangement means that the extreme compactness necessary for automotive use cannot be attained. This is a serious disadvantage not only for the reason that space limitations are of vital importance in automobiles but also for the reason that parallel input shafts require extra gears and greater complication with consequent high cost of manufacture. Whereas the cost consideration is of substantial significance in heavy duty transmissions, it is a primary if not overriding consideration in automotive transmissions.
In addition, any practical transmission must be capable of achieving a reasonable final output speed usually the same as the input speed or essentially a 1:1 ratio and it being understood that variations in vehicle weight, power, usage etc. are normally provided for by selection of rear axle gear ratios. In automotive usage, the final output ratio represents the most used condition of operation and it is preferred that the construction be the most efficient and long lasting at or near the 1:1 ratio.
In systems such as exemplified by the Ross patent where a single planetary gear set is used as a locked coupling in one range and with dual-input drive in the other range certain disadvantages ensue in connection with the overall operation. Most apparent is that in low range the planetary is direct coupled and does not function to multiply torque at maximum torque output operation. This requires the input gear reduction in single input drive as above mentioned. In high range the planetary functions to overdrive the output with respect to the dual drive and with the addition of the single drive input results in a substantial overdrive of the planetary output which is undesirable due to dynamic loadings on the planetary gear set and also requires an additional gear reduction beyond the transmissions in order to achieve an approximate 1:1 ratio.
In my prior patent and its Reissue, a low speed planetary gear set is used as such in reverse and the low speed mode and a high speed planetary gear set is used with dual-input drive in the high speed mode. Consequently, the general arrangement provides torque multiplication characteristics of a low range planetary gear set with single input drive coupled with a differential dual-input synchronous shiftable high speed planetary gear set offering substantial advantage from the practical design point.
If, in addition, such an arrangement could be so constructed as to be economical of manufacture while also obtaining a high degree of mechanical efficiency, a powerful tool for achieving a practical vehicle transmission meeting present and projected emission standards would be obtained. That is to say, the continuously variable drive ratio characteristic coupled with high mechanical efficiency would allow the most efficient use of power plants which are capable of meeting stringent emission standards.